Graphite, which consists predominantly of the element carbon, is used as a moderator in a number of nuclear reactor designs, such as the MAGNOX and AGR gas cooled reactors in the United Kingdom, and the RBMK design in Russia. During construction, the moderator of the reactor is usually installed as an interlocking structure of graphite bricks. At the end of reactor life, the graphite moderator, typically weighing about 2,000 tons, is a form of radioactive waste that requires safe disposal. Graphite is a relatively stable chemical form of carbon, which is in many ways suitable for direct disposal without processing. However, after neutron irradiation, the graphite will contain stored Wigner energy. The potential for release of this energy needs to be accommodated in any strategy which relies on disposing of the graphite in unprocessed form. Alternatively, processing the graphite before disposal can allow the safe release of any stored Wigner energy.
The graphite also contains significant quantities of radionuclides from neutron induced reactions, both in the graphite itself and in the minor impurities which it contains. Because of the structure of graphite, which includes loosely packed foliates or layers, the radioisotopes can become trapped within the spaces or pores of the graphite. The radioisotope content can conveniently be divided into two categories—short-lived isotopes and long-lived isotopes. Short-lived isotopes (such as cobalt-60) make the graphite difficult to handle immediately after reactor shutdown, but they decay after a few tens of years. Long-lived isotopes (principally carbon-14 and chlorine-36) are of concern through the possibility of their discharge to the biosphere. Carbon-14 is produced in the graphite in one of two ways. One way is the activation of nitrogen gas, with the carbon-14 present in pores of the graphite as carbon dioxide gas. The second way is through the neutron activation of carbon-13, which is a natural, stable isotope of carbon, making up just over 1 percent of the carbon in the graphite. Carbon-14 produced in this way would be part of the graphite matrix. Chlorine-36 is formed in a similar manner by irradiation of chlorine left in the graphite matrix during the graphite sintering process. Processing the graphite offers the opportunity to separate the majority of the graphite mass (carbon) from the long-lived radioisotopes. This processing in turn facilitates disposal of the graphite waste shortly after the end of the reactor life, and may permit recycling.
Because of the characteristics of graphite and its mass, the most common procedure to date for decommissioning graphite-moderated reactors is to store the reactor core in-situ for a period of tens of years following reactor shut-down. During this period, short-lived radioisotopes decay sufficiently to allow eventual manual dismantling of the graphite moderator. Most plans then assume that the graphite will be disposed of in its existing chemical form, with appropriate additional packaging to prevent degradation or release over the long period of carbon-14 and chlorine-36 decay.
Storage has certain negative consequences, such as the following: 1) an implication of long-term financial liability, 2) a visually intrusive storage structure that has no productive purpose, and 3) a requirement imposed on a future generation (which gained no benefit from the original asset) to complete eventual clearance. If the storage alternative is to be replaced by shorter term management, it is essential for the graphite to be processed in a safe and radiologically acceptable manner.
Certain prior techniques for treating radioactive graphite applied heat and oxidizing gases to treat the graphite in order to remove a sufficient fraction of the long-lived radionuclides within the graphite. These processes have shown that heating or “roasting” with inert gases, such as nitrogen or argon, alone can remove substantially all the hydrogen-3 (tritium) but the process cannot remove more than about sixty (60) percent of the carbon-14. Alternative processes have been performed to improve the carbon-14 removal by adding limited quantities of oxygen containing gases to the inert gas to provide oxygen that can preferentially convert the carbon-14 to carbon monoxide or carbon dioxide gases, that then can removed from the graphite. Testing with inert gases and oxygen containing gases (steam, carbon dioxide, nitrous oxide, oxygen) has shown that improved carbon-14 removal is possible but the presence of oxygen tends to dramatically increase the gasification of the bulk graphite. To reduce this gasification effect when oxygen containing gases are combined with the inert gases, the operating temperature of the roasting process must be reduced or limited to prevent excessive bulk graphite gasification. Unfortunately, by reducing or limiting the roasting temperature, the amount of carbon-14 removal is also greatly reduced or limited. As a consequence, when oxygen containing gases are introduced with the inert gases, the concentration of these oxidizing gases must be lowered so that higher temperatures can be used. Still, when roasting temperatures exceed approximately 1200° Celsius, the amount of bulk graphite gasified is excessive regardless of the reduced concentration of oxygen containing gases that are used.
The results of testing of these processes demonstrate that, if the concentration of oxygen containing gases is limited sufficiently to reduce bulk graphite gasification at temperatures greater than approx. 1200° Celsius, then the carbon-14 removal is greatly reduced to less than approx. sixty (60) percent, which is unsatisfactory. If the oxygen containing gas concentration is increased such that carbon-14 removal is satisfactory then too much bulk graphite is gasified. In either case, an objective of volatilizing more than ninety (90) percent of the carbon-14 while simultaneously reducing bulk graphite gasification to less than five (5) percent by weight cannot be achieved with these conventional methods.
What is needed are systems and methods that can subject the graphite to a sufficient temperature range to volatilize the radionuclides without gasifying the bulk graphite and specifically systems and methods that can remove greater than 90 percent of the carbon-14 while gasifying less than 5 percent of the bulk graphite.